Until now, as a production method for a polyacetal copolymer, a cationic copolymerization with trioxane as a main monomer, and a cyclic ether and/or cyclic formal having at least one carbon-carbon bond as a comonomer has been known. As the cationic active catalyst used for these copolymerizations, a Lewis acid, in particular halides of boron, tin, titanium, phosphorous, arsenic, and antimony, for example boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorous pentachloride, phosphorous pentafluoride, arsenic pentafluoride, and antimony pentafluoride, and their complex compounds or salts; protonic acids, for example perchloro acids; esters of protonic acids, in particular esters of perchloro acids and lower aliphatic alcohols, for example perchloro acid-tertiary butyl ester; anhydrides of protonic acids, especially mixed anhydrides of perchloro acids and lower aliphatic carboxylic acids, for example acetyl perchlorate, or trimethyloxoniumhexafluorophosphate, triphenyl-methylhexafluoroacetate, acetyltetrafluoroborate, acetylhexafluorophosphate, and acetylhexafluoroarsenate, and the like have been proposed. Among these, boron trifluoride or coordination compounds of boron trifluoride and organic compounds, for example ethers, are the most common as polymerization catalysts with trioxane as the main monomer, and are widely used industrially.
However, with commonly used polymerization catalysts such as boron trifluoride type compounds, in the later phase of polymerization the polymerization speed suddenly decreases, and it is nearly impossible to obtain polymer conversion ratio of near 100% in a short time, and a very long time is required, which is inefficient, and moreover, in the later phase of polymerization, decomposition of the generated polymer by the catalyst becomes relatively dominant, which not only causes a reduction in molecular weight, but also has the effect of degrading the qualities such as heat resistance and the like. Further, if the amount of the polymerization catalyst is increased, overall the polymerization speed is enhanced, and the polymer conversion ratio is also increased, but the quality of the generated rough polymer degrades more and more, and in a later step a complex stabilization treatment is required, whereby the production operation overall is by no means a preferable method.
Accordingly, the technique of adding a solution comprising a catalyst deactivation agent at a stage where the polymer conversion ratio is relatively low to stop the polymerization, then washing, recovering and purifying and reusing the remaining unreacted monomers, is widely practiced.
Further, there have been various proposals for improving the polymerization apparatus and the method of supplying the catalyst, in addition to increasing the polymer conversion ratio. For example, a technique of increasing the polymer conversion ratio for a device by inclining the polymerization device 1 to 10° (Patent Document 1), a technique of providing a weir at the discharging port of the polymerization apparatus (Patent Document 2), as well as a technique of mixing the catalyst with the comonomer in advance, and supplying this to the trioxane (Patent Documents 3 and 4) have been proposed. These techniques are all effective for increasing the polymer conversion ratio for the case of using a boron trifluoride type polymerization catalyst.
Further, a method has also been proposed to deactivate the rough polymer in the later stage of the polymerization by using a high activity, non-volatile polymerization catalyst, to directly recover the unreacted monomer without carrying out washing, and reuse the same (Patent Document 5). In this technique, it is possible to carry out recovery of the monomers directly from the rough polymer before deactivation, which was difficult with the boron trifluoride system commonly used in the prior art, and moreover, secondary reactions at a stage where a high conversion ratio has been achieved do not readily occur compared to a boron trifluoride type polymerization catalyst, whereby it is possible to obtain a rough polymer which is excellent in thermal stability with little unreacted monomers by extremely simple steps.
Patent Document 1: Japanese Examined Patent Application Publication No. H05-008725
Patent Document 2: PCT International Publication No. WO1996/013534
Patent Document 3: Japanese Unexamined Patent Application, Publication No. H11-255854
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H11-124422
Patent Document 5: Japanese Unexamined Patent Application, Publication No. H09-278852